mechanical engineering Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, an ...

, a crosshead is a

mechanical joint A mechanical joint is a section of a machine which is used to connect one or more mechanical part to another. Mechanical joints may be temporary or permanent, most types are designed to be disassembled. Most mechanical joints are designed to all ...

used as part of the

slider-crank linkage A slider-crank linkage is a four-link mechanism with three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement the slider, or the expansion of gases against a sliding piston in a cylinder ...

s of long

reciprocating engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fea ...

s (either

internal combustion An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combust ...

or steam) and reciprocating compressors to eliminate sideways force on the piston. Also, the crosshead enables the connecting rod to freely move outside the cylinder. Because of the very small bore-to- stroke ratio on such engines, the connecting rod would hit the cylinder walls and block the engine from rotating if the piston was attached directly to the connecting rod like on trunk engines. Therefore, the longitudinal dimension of the crosshead must be matched to the stroke of the engine.

Overview

On smaller engines, the connecting rod links the piston and the crankshaft directly, but this transmits sideways forces to the piston, since the

crankpin A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the ...

(and thus the direction the force is applied) moves from side to side with the rotary motion of the crank. These transverse forces are tolerable in a smaller engine. A larger engine's much greater forces would cause an intolerable degree of wear on the piston and

cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infin ...

, as well as increasing overall friction in the engine. A

piston rod In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels. Internal combustion engines, and in particular all current automobile engin ...

is attached to the piston and links it to the crosshead, which is a large casting sliding in crosshead guides (UK: ''slidebar''), allowing it only to move in the same direction as the piston travel. The crosshead also houses the

gudgeon pin In internal combustion engines, the gudgeon pin (UK, wrist pin or piston pin US) connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves.Nunney, Malcolm James (2007) "The Reciproc ...

(US: ''wristpin'') on which the small end of the connecting rod pivots. In this way, the transverse forces are applied only to the crosshead and its bearings, not to the piston itself.

Internal combustion engines

Internal combustion engines using crossheads make for easier maintenance of the top end of the engine, since the pistons can be easily removed. The piston rod is mounted on the underside of the piston and connected to the crosshead by a single nut in double acting engines. The large two-stroke

marine diesel engine Marine propulsion is the mechanism or system used to generate thrust to move a watercraft through water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electri ...

s are usually of this pattern. A crosshead is essential in a double-acting diesel engine (see also: battleships: 12 MAN double-acting 2-stroke 9-cylinder diesels). Large diesels often have a plunger oilpump directly attached to the crosshead to supply oil under high pressure to the crosshead bearing.

Steam engines

In the case of the steam engine, a crosshead is essential if the engine is to be ''double acting'' - steam is applied to both sides of the piston, which requires a seal around the piston rod. An exception is the

oscillating cylinder steam engine An oscillating cylinder steam engine (also known as a wobbler in the US) is a simple steam-engine design (proposed by William Murdoch at the end of 18th century) that requires no valve gear. Instead the cylinder rocks, or oscillates, as the ...

which can be double acting and yet has no crosshead.

Locomotives

Crossheads in a steam locomotive can be mounted either to one guide mounted above the crosshead or to two, one above and one below (called an ''alligator crosshead'' since it has two "jaws"). The former was preferred in many modern locomotives.

Marine engines

In many 19th century

marine steam engine A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their ...

s, the crosshead was a strong metal bar attached to the piston rod and perpendicular to it, which was sometimes used to eliminate transverse forces, as in a steeple engine, and at other times used as a linkage—to side-rods in a

side-lever A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their ...

engine or to connecting rods in a

square In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90- degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length a ...

engine.

References